NZ Rail Maps: Using Gimp To Georeference Retrolens Aerial Photos [1]: Introduction

OK so here is another post in the NZ Rail Maps category and this post will explain how I carry out the georeferencing of the historical Retrolens aerial photos to produce the maps for the NZ Rail Maps project. This is a very rewarding aspect of the project but it does take a lot of work to complete, and whilst it is my desire to have the maps produced for the majority of stations in NZ being documented historically, it may not be practical to achieve this for more than a few hundred locations in practice.
This is the first of several posts that will outline the various steps needed for this particular process and this particular post introduces the process,
The first question to be resolved is whether to use this system at all. There have been in QGIS software and possibly other GIS packages, tools or plugins provided that enable georeferencing directly within the GIS. The plugin for QGIS is poorly designed in my view and I had considerable difficulty in making it work. It is not fully visual in that it is not possible to preview the final result, so it does have limitations. I had previously experimented with the image overlay capability in Google Earth but found that it was limited in its design.
So using Gimp has been arrived at over a period of time, due to its being best supported. The main downside of using Gimp is the large files that are generated by it. These vary, but for example to cover about 10 km of track distance with one generation of historical aerial imagery can result in about a 20 GB disk file in order to get a desirable resolution for picking up all the details needed for trackwork. This has largely been arrived at due to the source resolutions available. NZR had their own surveys done for major stations and also major corridors, and these have scales varying from  1:3000 up to 1:5500, which gives enough detail to be able to count individual sleepers. Most station surveys are at 1:4350 approximately but there are a few around Auckland and Wellington that go as high as 1:3000. Corridor surveys to date have always been 1:5500 but there are some continuous station surveys covering 10 or 20 km of a corridor around Auckland and Wellington at the larger scales mentioned.
Where NZR surveys are unavailable then the next best choice generally will be either for some larger urban areas, urban surveys done for a local authority or some other government department at various scales, or where the rail line passes close to a highway corridor, then the highway surveys generally around 1:8000 will be adequate. Generally we are looking for a scale of 1:10000 minimum to get enough detail to pick out individual tracks in a station yard. Sometimes I use smaller scale surveys if there is nothing else available but they are very fuzzy looking to print as a map background and therefore have limited value.
In order to get the best out of surveys that are 1:10000 or larger scale we must also use base aerials that are able to match this scale. Base aerials are downloaded from the Linz data service and generally I am using the latest release for any particular area but I might also have the ability to use earlier versions through WMTS when drawing the maps in the GIS. However as we are talking about static imagery as a background in Gimp, I am going to download the latest version and for an area where I have historical imagery with a scale of 1:10000 or greater, I want to be using backgrounds that have a pixel size not exceeding 0.15 metres a side. But if there is only 0.3 metre imagery available for the area then scaling this to the size needed to get the smaller sized pixels is what has to be done. For most major urban areas there is generally imagery at 0.125, 0.1 or 0.075 metres resolution available and no scaling is needed, but generally outside these regions, 0.3 metre imagery will have to be scaled up to double in each dimension, which Gimp handles by doubling the pixels in each direction, effectively cutting each existing pixel into four pieces. It is then necessary when importing these new larger tiles into Qgis to change the settings in the world file to reflect the new pixel size correctly so that the tiles fit into the original tile grid. This will be covered in a future part of the series.
So there are basically two important steps to be determined first of all when embarking on this process. The most significant of these is to ascertain what is available from Retrolens or any other source for the historical imagery, and then from there, the second step is to get suitable base imagery from Linz Data Services that can provide the georeference base for the historical imagery. The next step covered in Part 2 is to prepare both the base and historical imagery for georeferencing.